The open-end correction of an acoustic tube has been widely investigated in a free-field condition (cf. Hall, 1987). However, no experimental data give accurate open-end corrections for side branches within an acoustic tube, although some proposals have been made [see Fant (1960)]. To incorporate side branches such as the paranasal sinuses and piriform fossa in an acoustic model of the vocal tract, an end correction coefficient is definitely needed. Thus this study estimated the end correction coefficient of side branches experimentally and computationally. A number of acoustic tubes were used to examine effects of (1) the angle from the branch to the main tube, (2) the distance between the open end and the opposite tube wall, and (3) the geometric shape around the open end of the branch. The results indicate that the end correction coefficient depends on the dimension in front of the open end, and is independent of the angle of the branch to the main tube. The open-end correction coefficient (C[inf e]) of a branch can be described by the diameter (D) of its open end and the distance (L) from the end to the opposite tube wall as shown: C[inf e]=0.414+0.517 xlog(L/D[sup 2]), for 0.158<(L/D[sup 2])<6.24. Application of this empirical formula to actual vocal geometries will be discussed.